The present invention relates to radio frequency (RF) receive coils and magnetic resonance scanners incorporating them. The invention finds particular application in isolating or detuning an RF excitation coil during portions of an imaging process in which it does not participate. It is to be appreciated however, that the present invention may find further application in isolating individual coil portions of a single coil, isolating reserve coils during excitation, selected among plural coil resonance frequencies, or in other arts in which selective RF signal reception is practiced.
Magnetic resonance imaging (MRI) machines apply a main magnetic field through an examination region. This strong field, typically denoted B0, acts to align the nuclei within a subject to be examined. In some MRI machines, the B0 field is horizontally oriented, and in others it is vertically oriented.
In both horizontally and vertically oriented systems, magnetic resonance is excited in the aligned nuclei by a relatively strong orthogonal RF field, typically denoted B1. The B1 field causes the aligned nuclei or spins to tip into an plane orthogonal to the static magnetic field B0. Over time, the spins realign themselves with the B0 field emitting relatively weak radio frequency (RF) resonance signals as they precess. This resonance is detected by RF coils tuned to the specific resonance frequency desired. These resonance signals are passed to image processing equipment to reconstruct the signals into an image representation for display on a video monitor.
The transmitted excitation RF pulses and the received resonance signals are in the same, limited frequency spectrum. Hence, the same coil is often used for both transmission and reception. However, in some applications, it is advantageous to have separate transmit and receive coils. When two coils in the same system are tuned to the same frequency, they tend to couple and otherwise interact detrimentally.
Typically, the transmit RF signals are orders of magnitude larger than the magnetic resonance signals generated by the excited nuclei and detected by the RF receive coils. To ensure optimum receiving conditions, the transmit coil is typically detuned during the reception mode. This eliminates the loss of RF resonance power to the transmit coil during receiving. This lessens the chance of noise reception in the form of gradient pulse energy retransmission by the transmit coil in the resonance frequency spectrum. Accordingly, it is advantageous to desensitize the transmit coil to the Larmor frequency of the resonating dipoles so it does not negatively affect the receive process.
Typically, the resonance frequency of the transmit coil is changed by connecting one or more capacitors in parallel to or in series with the coil through a PIN diode or diodes. The PIN diodes are controlled by a DC bias applied by separate control electronics and control cables. When the PIN diode is set to conduct by release current, the series connected capacitor becomes electrically connected as part of the coil circuit. When the direct current is switched off, the resistance of the PIN diode grows high enough to electrically disconnect its series connected capacitor from the coil circuit. Control cables for the PIN diodes act as RF antennas and conductors carrying stray resonance frequency RF signals into the system. A lowpass filter is used to lock the RF resonance frequency in each of the control cables. The separate PIN diode control electronics, DC biasing potentials, and cabling impose restrictions on the design and shape of the transmit coil, as well as restrictions on the placement of PIN diodes in the coil structure.
The present invention contemplates an improved method and apparatus to detune a transmit coil which overcomes the above-referenced problems and others.
According to one aspect of the present invention, a magnetic resonance imaging apparatus is provided. Gradient coils spatially encode a main magnetic field generated by the apparatus. Radio frequency transmit coils transmit RF pulses as directed by an RF transmitter, an RF control processor, and a tuning synthesizer into an imaging region exciting and manipulating magnetic dipoles therein. The transmit coils include tuning circuits which shift the resonance frequency of the coils when activated. At least one radio frequency receive coil receives magnetic resonance signals, which are demodulated by one or more receivers, and reconstructed by a reconstruction processor.
According to a more limited aspect of the present invention, the tuning circuit includes a tuning capacitor, a biasing power supply, and a circuit element, the circuit element including a PIN diode, and the biasing power supply including a resistance and a rectifying diode.
According to another aspect of the present invention, an RF transmit coil is given. The RF transmit coil contains conductive, inductive, capacitive, and reactive elements that tune the RF transmit coil.
According to another aspect of the present invention, a method of magnetic resonance imaging is given. A main magnetic field is generated in an imaging region and spatially encoded. Magnetic resonance is excited and manipulated with an RF transmit coil, and received with an RF receive coil and demodulated. The RF transmit coil is detuned while the magnetic resonance signals are being received.
According to another aspect of the present invention, a detuning circuit is given. The detuning circuit uses PIN diodes and capacitances to change the resonance frequency of an RF transmit coil.
One advantage of the present invention resides in the ability to detune an RF transmit coil during a receive phase of a magnetic resonance imaging event.
Another advantage of the present invention is that it requires no added hardware.
Another advantage of the present invention is that it allows for the elimination of selected cabling and circuitry from a typical MR system while maintaining the integrity of the system.
Another advantage of the present invention is that it reduces imaging artifacts.
Another advantage of the present invention is that it grants greater freedom for transmit coil designs.
Yet another advantage of the present invention is that it minimizes power loss from the resonance signals.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.